This invention relates to the excavating arts, and more particularly, to improvements in removable tooth/lip configurations such as may be used in the buckets of bucket wheel excavators or the like.
One environment in which the present invention finds great utility is in the mining of tar sands which are very difficult to work, particularly in harshly cold environments.
Tar sands (also known as oil sands and bituminous sands) are sand deposits which are impregnated with dense, viscous petroleum. Tar sands are found throughout the world, often in the same geographical area as conventional petroleum. The largest deposit, and the only one of present commercial importance, is in the Athabasca area in the Northeast of the Province of Alberta, Canada. This deposit contains over 700 billion barrels of bitumen. For comparison, this is about one-sixth of the U.S. coal reserves. It is just about equal to the known world oil reserves.
Tar sand has been defined as sand saturated with a highly viscous crude hydrocarbon material not recoverable in its natural state through a well by ordinary production methods. Strictly speaking, the material should perhaps be called bituminous sand rather than tar sand since the hydrocarbon is a bitumen (carbon disulfide soluble oil). In petroleum refining, tar is a term reserved for the residue of a thermal process. The term oil sand is used also, possibly in allusion to the synthetic crude oil which can be manufactured from the bitumen.
It has long been realized that tar sand is a mixture of sand, water, and bitumen. The sand component is predominantly quartz, in the form of rounded or sub-angular particles. Each grain of sand is wet with a film of water. Surrounding the wetted sand grains, and somewhat filling the void volume among them, is a film of bitumen. The balance of the void volume is filled with connate water, and, in some instances, a small volume of gas. The gas is usually air, but some test borings in the Athabasca deposit have reported methane. The sand grains are packed to a void volume of about 35%. This corresponds to a tar sand mixture of roughly 83 wt.% sand, the balance being bitumen and water. In fact, it is found with considerable regularity, that the bitumen and water weight percentages total about 17% of the tar sands.
There are two basic approaches to recovering bitumen from tar sand. The tar sands may be mined and transported to a processing plane where the bitumen is extracted and the sand is discharged. Alternatively, the separation of bitumen from sand may be accomplished without ever moving the sand, that is, in situ. In situ processes have a great deal in common with tertiary recovery of conventional crude oil. To date, no commercial in situ operations have been developed in the Athabasca region although numerous development projects are being studied.
Almost 200 years after the discovery of the oil bearing sands by Peter Pond, the Great Canadian Oil Sands, Ltd., (GCOS), a subsidiary of the Sun Company of Philadelphia, Pa., was the first company to take the risk of producing synthetic crude oil from oil sands. In 1962, the Alberta Provincial Government granted GCOS the prospecting rights for Lease IV, about 32 kilometers north of Ft. McMurray on the banks of the Athabasca River. This is one of the areas with the smallest overburden thickness such that the tar sand can be fairly readily mined and transported from the formation to a processing plant. In order to support synthetic crude production in a plant of economical size, an immense mining operation is called for. For instance, the GCOS project was designed to produce 45,000 barrels per calendar day of synthetic crude for which a tar sand mining rate of about 100,000 tons per day is necessary (In fact, these figures are now routinely exceeded.) This refers to the tar sand (ore) only and does not include the overburden which must be stripped away in order to expose the tar sand for mining. This mining rate is of the order of the size of the largest mines in North Amercia. Because of the large scale of mining involved, only open pit methods have been commercially employed for exploiting the Athabasca tar sands.
Because of the relatively low unit value of tar sand as an ore, mining and transportation costs must be rigorously minimized. This means, among other things, that the feed must receive only a minimum amount of handling between the mine and the processing plant. On the other hand, for economy of operation in the processing plant, continuous units must be designed to operate with a relatively steady feed rate, round the clock. While such processing is usual in petroleum refineries, it is very definitely the exception in mining. Thus, quite apart from the relatively large scale, tar sand mining presents the unique problem of assuring a relatively steady feed rate to the processing plant, round the clock and year around. In addition to this requirement, which is general for any tar sand formation, the Athabasca tar sands present two other significant problems for mining; viz.: (1) tar sand in-place requires very large cutting forces and is extremely abrasive to the cutting edges exposed to it; and (2) both the equipment and pit layouts must be designed to operate during the long Canadian winters at temperatures as low as 60.degree. F. below zero.
Basically, there are two approaches to the open pit mining of tar sands. The first is to use a few mining units of custom design, which will necessarily be very expensive. For instance, large units which have been considered are bucket wheel excavators, dredges (both hydraulic and bucket ladder) and super-sized drag lines. The other approach is to use a multiplicity of smaller mining units of conventional design and relatively much lower unit cost. For example, scrapers and truck-and-shovel operations have been considered. Each method has advantages and its own peculiar risks. The former method has been adopted by GCOS and will be followed by the even larger Syncrude project which is located a few miles from the GCOS plant and which is scheduled to commence commercial operation in 1978.
In the GCOS mine, the ore body is divided into two layers or benches, each nominally 75' in height. The pit floor and the dividing plane between the upper and lower bench are roughly horizontal. Mining of tar sands is carried out by two giant bucket wheel excavators, and the overburden is removed by a third giant bucket wheel excavator. Tar sands loosened from the face of each bench by the bucket wheel are discharged onto a crawler-mounted conveyor or belt wagon which in turn discharges the ore onto movable conveyors. These conveyors are advanced from time to time in the direction of mining. The movable conveyors discharge onto trunk conveyors which in turn feed a main conveyor. The main conveyor carries the tar sand ore to the processing plant where it is converted to synthetic crude oil.
Each of the tar sands bucketwheel excavators in operation at GCOS is capable of mining in excess of 9,000 tons per hour, more than enough to feed the entire plant. However, those knowledgeable in the art will appreciate that tar sands is a very difficult medium to work, particularly in hostile environments such as that experienced in the Athabasca region during the winter months. As a result, very careful attention has been given to the development of bucket wheel buckets employing carefully configured and distributed teeth which are readily individually removable for replacement after they have become worn.
In the open pit mining of tar sand, one procedure which may be followed is to slue the buckets across the face of the work (i.e., to move them laterally or sideways across the working face), to thereby take a substantially horizontal cut of the sands. Thus, the actual cutting is primarily carried out by teeth disposed on the sides of the bucket. In the best prior art configuration, a tooth-mounting bucket wheel has a variable profile, formed by the relieving of the center of the lip in both planes. The lip member is of generally U-shaped configuration, viewed head-on or looking into the bucket, and is skewed rearwardly at its two opposite ends. The end face of the lip member is provided with a plurality of sockets for individually receiving the shanks of chisel-shaped excavating teeth, and the member has means for removably securing the teeth in position in their respective sockets. For a more complete description of such prior art system, one may refer to U.S. Pat. No. 3,791,054 entitled "Lip Construction for Bucket Wheel Excavators" issued Feb. 12, 1974 and corresponding Canadian Pat. No. 941,861, issued Feb. 12, 1974, both assigned to Great Canadian Oil Sands, Ltd. Briefly, however, it may be noted that the bucket wheel system disclosed in these references employ removable teeth having oval-in-cross section shank portions which fit into complementarily configured apertures distributed about the bucket lip. Because the time spent changing out these teeth is substantial and to the detriment of the ore throughput in the mine and because the wear observed both to the teeth shanks and, more importantly, to the teeth receiving sockets in the bucket lips has proven considerable, it will be readily apparent to those skilled in the art that it would be highly desirable to provide a bucket tooth/lip configuration by which the period during which a set of teeth may remain in operation is substantially extended while, simultaneously, improved wear characteristics are imparted to the system in order that replacement teeth will continue to fit rigidly into the sockets about the bucket lip.
A related and very serious problem which has been observed during the use of the prior art bucket lip/removable tooth system is a tendency for overstressed teeth to fracture at the base of their distal portions. When such fracture occurs, the bucket lip must directly address the material being mined and begins to wear rapidly. Consequently, replacement teeth will not fit correctly, and the lip geometry is altered, conditions which quickly render a bucket virtually useless. It then becomes necessary to change out the entire bucket with a heavy penalty, not only in capital investment, but in down time. It will therefore be understood by those skilled in the art that it would be a most important step forward in the art to provide means by which tooth fracture, if it occurs, takes place outboard from the base of the tooth distal portion in order that the bucket lip will remain shielded from the material being mined.
It is therefore a broad object of our invention to provide improved means for mining abrasive and difficult to work ore materials.
It is another object of our invention to provide in an excavating system, a removable tooth/bucket lip configuration by which teeth fitted to a bucket may remain in service for extended periods of time.
It is yet another object of our invention to provide such a removable tooth/bucket lip configuration in which each tooth is rigidly maintained in order to avoid wear between the shank portion of the tooth and the bucket lip tooth receiving socket.
In a more specific aspect, it is an object of our invention to provide a removable tooth/bucket lip configuration in which the teeth are provided with rectangular-in-cross section shank portions and forwardly angled shoulder means for insertion into a complimentarily configured socket in the bucket wheel lip.
In another aspect, it is an object of our invention to provide such a removable tooth/bucket lip configuration in which tooth fracture, it it occurs, takes place outboard from the base of the tooth distal portion.